Fix: libcrmcommon: Extend what pcmk__split_range can do.

• Handle if the input string consists solely of the separator (return

false, start & end are NULL).

• Handle a range with no starting value (return true, start is NULL, end

is some value).

• Handle a range with no ending value (return true, start is some value,

end is NULL).

• Add test cases for the above.
• Be a little more explicit about supporting the other cases.

The cron_check macro in lib/pengine/rules.c has been updated to take
into account the possibility that a range with no start is now a valid
possibility.